1. Field of Invention
The invention relates generally to frequency modulation of an RF carrier and more particularly to frequency modulation calibration in a phase-locked loop.
2. Description of the Prior Art
It is well known in the art to synthesize frequencies by means of phase-locked loop circuits. The phase-locked loop (PLL) includes a tunable oscillator (typically a voltage-controlled oscillator (VCO)) whose output is locked to a known reference signal by means of a phase comparator. The phase comparator generates an output voltage or current that is proportional to the phase difference between the two signals. The phase comparator output is fed back to the input of the VCO and used to tune the VCO. This forces the VCO output to have exactly the same frequency as the reference signal. By interposing a divide-by-N block in the circuit comparator, the reference frequency may instead be compared with the VCO frequency divided by N; the VCO output will then be locked to N times the reference frequency. By varying N, it is possible to generate frequencies which are the Nth harmonics of the reference frequencies, where N is an integer. Another technique, called fractional N, makes it possible to generate frequencies that are any rational multiple of the reference frequency. Such a technique is disclosed in U.S. Pat. No. 3,928,813 issued to Charles A. Kingsford-Smith on Dec. 23, 1975, entitled "Device for Synthesizing Frequencies which are Rational Multiples of a Fundamental Frequency".
In some applications, it is often desired to frequency modulate (FM) the synthesized signal. A PLL is in effect a control system that maintains a constant phase difference between two signals. Any variations in the phase of one signal relative to the other are removed by the PLL. This property of a PLL is utilized to suppress noise and clean up a signal; however, this property also tends to suppress frequency modulation of a signal in PLL.
Audio FM may be accomplished by splitting the FM signal into two separate signal paths. One path is AC-coupled to the VCO and will be the primary path for the FM signal for frequencies that are above the bandwidth of the PLL. FM at frequencies inside the PLL bandwidth is integrated and then injected into a summing node at the output of the phase detector. Since phase is the integral of frequency, FM at frequencies within the PLL bandwidth is accomplished by phase modulation (PM). Properly scaling the gains of each signal path provides flat FM response both inside and outside the PLL bandwidth.
One of the primary problems encountered when frequency modulating a signal in a PLL is that the gain of the VCO is not constant--generally the gain of the VCO varies with the frequency of the VCO. One solution to this problem is to FM only at a single VCO frequency. Another solution injects an FM calibration signal into the loop through a variable-gain amplifier, then the gain of the amplifier is varied to match the frequency demodulated VCO output signal to a reference signal.